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Visible-light photocatalytic activity of TiO_xN_y thin
films obtained by reactive multi-pulse High Power
Impulse Magnetron Sputtering
Alexandra Demeter, Florentina Samoila, Vasile Tiron, Dana Stanescu, Helene
Magnan, Mihai Straticiuc, Ion Burducea, Lucel Sirghi
To cite this version:
Alexandra Demeter, Florentina Samoila, Vasile Tiron, Dana Stanescu, Helene Magnan, et al..
Visible-light photocatalytic activity of TiO_xN_y thin films obtained by reactive multi-pulse
High Power Impulse Magnetron Sputtering.
Surface and Coatings Technology, Elsevier, 2016,
Visible-light photocatalytic activity of TiO
x
N
y
thin
films obtained by
reactive multi-pulse High Power Impulse Magnetron Sputtering
Alexandra Demeter
a, Florentina Samoila
a, Vasile Tiron
a, Dana Stanescu
b, Helene Magnan
b, Mihai Straticiuc
c,
Ion Burducea
c, Lucel Sirghi
a,⁎
a
Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics,“Alexandru Ioan Cuza” University, Iasi 700506, Romania
b
SPEC, CEA, CNRS, Université Paris-Saclay, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
c
Horia Hulubei National Institute of Physics and Nuclear Engineering, Magurele 077125, Romania
a b s t r a c t
a r t i c l e i n f o
Article history: Received 31 May 2016 Revised 30 September 2016
Accepted in revised form 3 October 2016 Available online xxxx
Reactive High Power Impulse Magnetron Sputtering operated in multi-pulse mode (m-HiPIMS) of a pure Ti tar-get in Ar/N2/O2gas mixture (massflow rates of 50, 2 and 0.16 sccm, respectively) has been used for the
deposi-tion of titanium oxynitride (TiOxNy) thinfilms with variable content of nitrogen (from 0.6 at.% to 24.2 at.%).
Increase of the nitrogen content in the deposited TiOxNythinfilms determined a decrease of the optical bandgap
energy and a corresponding increase of visible light adsorption. The photocatalytic activity for water molecule splitting of thefilms deposited on metallic substrate, which were used as the photo-anode in an electrochemical cell, has been investigated by measurements of photoelectrochemical current intensity versus biasing voltage during on/off cycles of visible light irradiation (sun light simulated by a xenon lamp). The as-depositedfilms have a short range order corresponding to rutile and anatase structures and showed very weak photocatalytic ac-tivity and chemical instability in the electrolyte of the photoelectrochemical cell. However, a post-deposition an-nealing treatment of thefilm with low content of nitrogen (0.6 at.%) improved considerably the visible-light photocatalytic activity, thefilm crystalline order and chemical stability.
© 2016 Published by Elsevier B.V. Keywords: Titanium oxynitride m-HiPIMS deposition Visible-light photocatalyst Water splitting 1. Introduction
Since the discovery of photocatalytic water splitting by Honda and Fujishima[1], titanium dioxide has been intensively studied as an envi-ronmental material with applications in photocatalytic purification of air and water and hydrogen fuel production from photocatalytic split-ting of water molecules[2]. Among other photocatalyst semiconductors, TiO2has been the most investigated material because, on one hand, it is
regarded as a theoretical material model of photocatalyst semiconduc-tors[3]and, on the other hand, it has very attractive properties for photocatalysis applications as low cost, high chemical stability, long life-time of electron/hole pairs and high oxidizing power of photogenerated holes[4]. However, the large bandgap of TiO2(3.2 eV for anatase and
3 eV for rutile)[5]restricts its good photocatalytic activity to a small fraction of solar energy radiation (UV light). Therefore, a large number of studies have been dedicated to extension of the photocatalytic activ-ity of TiO2to visible light. A good photocatalyst semiconductor for water
splitting under solar light should have the minimum energy of the con-duction band (CBM) higher than the potential energy for hydrogen re-duction (H+/H
2) and the maximum of the valence band (VBM) lower
than the potential energy of oxygen oxidation (O−/O2), which is at
−1.23 eV (versus the NHE). For anatase TiO2the CBM is just above
the H+/H
2reduction energy level (0.37 eV), while the VBM is located
deeply at −2.83 eV (versus NHE) [6]. Therefore, visible light photocatalysts developed from the TiO2structure used doping with
metal cations or acceptor anions for engineering the band energy struc-ture of this material. Compared to cation doping, the anions doping is less likely to form charge recombination sites and is, therefore, consid-ered a more effective approach used for enhancing the photocatalytic activity of TiO2. However, Yan et al.[6]have found that TiO2co-doping
with Zr cations and N anions is more effective than doping with either cations or anions. Asahi et al.[7]have theoretically evaluated the effect on the band energy structure of substitutional doping with C, N, F, P and S anions for O in anatase TiO2and found N as the best candidate for
en-gineering the TiO2energy band structure for visible light photocatalyst
applications. In N-doped TiO2, the p states of N mix with 2p of O, thus
shifting the VBM upwards without notably changing the position of CBM. Since the pioneering work of Asahi et al.[7], many authors tried to synthesize good photocatalyst semiconductors based on nitrogen doped titanium dioxide or titanium oxynitride (TiOxNy)[8]. However,
the fact that the states introduced by dopant atoms act as recombination centers for electron–hole pairs as well as the thermal instability associ-ated with doped materials humpered the photocatalytic performance of
Surface & Coatings Technology xxx (2016) xxx–xxx
⁎ Corresponding author.
E-mail address:lsirghi@uaic.ro(L. Sirghi).
http://dx.doi.org/10.1016/j.surfcoat.2016.10.011
0257-8972/© 2016 Published by Elsevier B.V.
Contents lists available atScienceDirect
Surface & Coatings Technology
these new materials. The photo-catalytic process is also strongly affect-ed by bulk material properties, such as crystallinity, a better crystallinity increasing the life time and mobility of charge carriers[9]. In general, the photocatalytic semiconductors with high defect density have a weak photocatalytic activity because of the formation of charge recom-bination centers. Recently, Yabi et al.[10]have predicted the Ti3O3N2
crystal as a new water-splitting photocatalytic material. The predicted value for instability energy,ΔH = 31 MeV/atom, indicates that this ma-terial can be synthesized. However, synthesis of this mama-terial has been not reported so far. The predicted band edge structure of Ti3O3N2is
in-teresting for visible light water-splitting photocatalysis because the CBM and VBM bracket the water redox levels and the energy bandgap is 2.37 eV. Due to this low value of the energy bandgap, the Ti3O3N2
crystal may exhibit better photocatalytic performance than TaON, the best known oxynitride photocatalyst so far[11].
In this work, we investigate the capability of the reactive High Power Impulse Magnetron Sputtering (HiPIMS) deposition technique to syn-thesize TiOxNythinfilms with good visible light photocatalytic activity
for water splitting. This deposition technique showed good reproduc-ibility and process stability for synthesis of oxides, nitrides or oxynitrides thinfilms[12], which otherwise are difficult to achieve by more conventional magnetron sputtering techniques[13,14]. In a previ-ous work[15], we have shown that reactive HiPIMS[16]working in multi-pulse mode (m-HiPIMS)[17]in Ar/O2gas mixture (0.2% O2of
the total massflow rate) can be easily manipulated for synthesis of substoichiometric TiOxthinfilms with x values down to 1.63. The deficit
of oxygen in the deposited TiOxfilms was controlled by the HiPIMS
pulse repetition frequency. Increase of this parameter determined a transition from oxidized towards metallic target sputtering with a no-ticeable increase of the sputtered Ti atoms in the gas phase. The limited amount of oxygen in the deposition chamber and the increased amount of sputtered Ti determined depositions of TiOxthinfilms with a large
deficit of oxygen at large values of the HiPIMS pulsing repetition rate. By adding nitrogen to the working gas, the excess of sputtered metal atoms and limited amount of oxygen content favour depositions of metal oxynitridefilms with larger content of nitrogen. Recently, we have proved this mechanism in the case of ZnOxNythinfilms. Here,
we use reactive m-HiPIMS depositions in Ar/O2/N2gas mixture for
syn-thesis of TiOxNywith the content of N ranging from 0.6 at.% to 24.2 at.%
(which is close to the N concentration in the predicted Ti3O3N2
materi-al). Structure and composition of depositedfilms have been investigat-ed by atomic force microscopy (AFM), X-ray diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption and photogeneration of electron–hole pairs under visible light irradiation were investigated by optical transmission spectroscopy measurements. The photocatalytic activity for water splitting under visible light illumi-nation has been investigated by measurements of photoelectrochemical current intensity variations versus biasing voltage during on/off cycles of visible light irradiation (sun light simulated by a xenon lamp). 2. Experimental devices, methods and techniques
In the m-HiPIMS, a single voltage pulse applied to the cathode (−700 V in peak value) with 15 μs in width was decomposed into a se-quence of 3 individual micropulses with the width of 5μs. By doing this, the sputtering rate has been noticeably improved due to diminished back attraction of sputtered and ionized atoms. The delay time between the micropulses within a sequence was kept constant (50μs) in all the experiments. The value of 50μs for the delay time between micropulses in m-HiPIMS has been chosen to optimize the m-HiPIMS deposition [17]. Typical time variations of the current intensity and discharge volt-age during m-HiPIMS operation at sequence repetition frequency values of 400 and 1600 Hz are given in Fig. 1. Apart of a decrease of preionization voltage from−310 V to −20 V, the voltage waveform does not change noticeably with the increase of the sequence repetition
frequency from 400 Hz to 1600 Hz. The role of the pre-ionization is to provide between discharge pulses certain ionization degree of the working gas (very weak plasma), which prevents giant impedance jumps when switching from insulator to conductor (plasma) gas state. It is worth mentioning that, at high sequence repetition frequency the pre-ionization is no longer required, its role being played by the residual charged species surviving in the afterglow plasma. The peak value of the current intensity corresponding to thefirst micropulse from the multi-pulse sequence is sensitive to the repetition frequency due to a change of target surface condition during the long time between consecutive micropulse sequences. At low repetition frequency the discharge operates in compound mode, the target surface being covered with oxynitride compound, which determines a lower value of the peak dis-charge current intensity for thefirst micropulse. As the repetition fre-quency increase, the period between sequences becomes smaller and the target surface becomes less poisoned. The average power increases linearly from 20 to 80 W as the sequence repetition frequency increases from 400 to 1600 Hz. By increasing the m-HiPIMS pulsing repetition fre-quency, the amount of sputtered Ti increases and, due to the limited amount of oxygen in the deposition chamber, depositions offilms with larger content of nitrogen is favoured.
All the experiments were performed in a high vacuum stainless steel chamber using a Ti target (99.995% purity, produced by Kurt J. Lesker Company) with a diameter of 50 mm and thickness of 5 mm. A low con-tent in oxygen of the working gas was chosen to facilitate incorporation of nitrogen in the deposited TiOxNythinfilms. Thus, the working gas
was a mixture of Ar (massflow rate of 50 sccm) and O2(massflow
rate of 0.16 sccm) and N2(massflow rate of 2 sccm) at the total pressure Fig. 1. Typical waveforms of the current intensity (a) and discharge voltage (b) for m-HiPIMS (sequence of three micropulses of 5μs in width separated by time-off periods of 50μs) at sequence repetition frequency values of 400 and 1600 Hz, respectively.
of 0.85 Pa. Use of a larger concentration of oxygen in the working gas determined depositions of pure TiOxthinfilms (negligible content of
ni-trogen) regardless the HiPIMS pulse repetition frequency. Incorporation of nitrogen in the as-depositedfilms was facilitated also by the use of room temperature for the deposition substrates. Increase of the sub-strate temperature determined a drastic decrease of nitrogen content of the depositedfilms. The TiOxNythinfilms were deposited on quartz,
glass and copper substrates placed at on the target axis at a distance of 50 mm from the target. The deposition rate was measured by a quartz crystal microbalance (QCM) placed beside the substrate. The thickness of the depositedfilms was monitored by the QCM and kept around 100 nm by adjusting the deposition time. Details on the set-up and m-HiPIMS technique have been given in a previous work[17].
The crystalline structure of the deposited thinfilms was investigated by an X-ray diffractometry and Raman spectrometry. The X-ray diffrac-tometer (LabX XRD-6000 from Shimadzu Co.) used Cu-Kα X-ray source (λ = 1.54059 A) in Bragg–Brentano configuration. The films were ana-lyzed in the range of 2θ = 10°–50°, with a scanning rate of 4°/min and a grazing incidence angle of 2°. The Raman spectrometer (LabRAM HR-800, Horiba-Jobin Yvon) was used in the back-scattering configuration to collect spectra at room temperature in the range 100–1000 cm−1of
thefilms excited by a 632 nm Ar+
laser line excitation. The scattered light was detected by a water-cooled charge coupled device detector.
The chemical composition of the depositedfilms was investigated by X-ray Photoelectron Spectroscopy (PHI 5000 VersaProbe XPS system from ULVAC PHI, Inc.) and Rutherford Backscattering Spectrometry (RBS) measurements. The RBS used a monochromatic (3.72 MeV)
4He2 +ions beam and non-Rutherford14N (4He,4He)14N scattering
cross-section resonance at 3.72 MeV for TiOxNysample analysis. The
measurements were performed in IBM geometry with an incident angle of 7°, in order to avoid channeling effects, and a laboratory scatter-ing angle of 165°. Scattered particles were registered with an AMETEK type BU-012-050-100 charged particle detector, having a solid angular acceptance of 1.641 msr and connected to a standard spectrometric chain and acquisition system. The typical energy resolution of the spec-trometer was 18 keV. The RBS spectra were evaluated using SIMNRA code.
The topography images and roughness of the depositedfilm surfaces have been obtained by atomic force microscopy (AFM) measurements performed in non-contact mode with a silicon AFM probe (NSG 03 from NT-MDT, Russia) with a sharpened tip (nominal curvature radius of 10 nm) and a stiff cantilever (resonant frequency of 150 kHz and force constant of 6.1 N/m).
The optical band-gap of the depositedfilms on quartz substrates were estimated from optical transmittance spectra obtained by a UV– VIS spectrophotometer. The photocatalytic activity of the deposited TiOxNythinfilms was characterized by photo-electrochemical (PEC)
re-sponse and methylene blue (MB) degradation measurements. The PEC response, light excitation efficiency and photo-sensitivity of TiOxNy
films deposited on copper substrates were tested in a conventional three electrode arrangement electrochemical cell with aqueous electro-lyte solution (0.1 M NaOH). Thefilms loaded on the PEC cell were irra-diated by an ozone-free Xe arc lamp mimicking the AM 1.5 solar spectrum at midday for mid-latitudes (100 mW/cm2in wavelength
range 200–1000 nm). The infrared radiation was filtered just after the lamp output to avoid inconvenient heating of the experimental setup.
Details on the experimental procedures and equipment for the PEC measurements have been described in[18]. For the MB decomposition experiments, thefilms deposited on quartz (1 × 1 cm2in size) were
im-mersed in aqueous MB solution (6 ml) and then irradiated by visible light (λ N 400 nm, incident light flux 120 mW/cm2homogeneously
dis-tributed over a surface area of 1.6 cm2) for a total time of 2 h. The
con-centration of MB solution was determined after every 5 min of irradiation by measurements with a UV–VIS spectrophotometer of the absorbance peak at wavelength 664 nm. The same quantity of MB solu-tion without any immersedfilm has been used as control.
3. Results and discussion
In this work, six TiOxNythinfilms (samples labeled from S1 to S6)
were successively deposited by m-HiPIMS at different values of pulse sequence repetition frequency on glass, quartz and copper substrates at room temperature. InTable 1are presented atomic composition, op-tical bandgap values, and the visual aspect (color) of the six samples. The following subsections present the results concerning investigations offilm structure and composition, and their optical and photocatalytic properties.
3.1. Structure, composition and optical bandgap
The atomic composition of the TiOxNythinfilms deposited on glass
substrates was determined by means of Rutherford Backscattering Spectroscopy (RBS). The results for six samples (S1 to S6) obtained by HiPIMS depositions at six values of pulse sequence repetition frequency are presented inTable 1. The increase of the m-HiPIMS repetition fre-quency from 750 to 1600 Hz determined an increase of the N content of the deposited TiOxNythinfilms from 0.6 at.% to about 24 at.%. The
value of N content in thefilm deposited at m-HiPIMS pulse repetition frequency of 1600 Hz is close to the N content in the stoichiometric Ti3O3N2compound (25 at.%). The mechanism explaining the effect of
m-HiPIMS repetition frequency on the nitrogen content of the deposit-edfilms is based on the fact that the sputtered titanium atoms are more reactive towards oxygen than towards nitrogen. The oxygen content of the working gas was set to a very low value (0.16%) to determine a def-icit of oxygen in the depositedfilms[15]. By increasing of m-HiPIMS repetition frequency, the amount of sputtered titanium atoms is in-creased, which means that a larger amount of titanium is available for reactions with nitrogen, after all the oxygen atoms from the deposition chamber were consumed.
The optical properties of thefilms were determined from their trans-mittance spectra recorded using an UV–VIS spectrophotometer. Optical transmittance spectra recorded in the wavelength range 300–1100 nm are shown inFig. 2. It is noticed that the optical absorption edge is shifted towards longer wavelengths with the increased content of nitro-gen in the as-depositedfilms, which at its turn increased with the in-crease of HiPIMS pulse sequence repetition frequency. The energy bandgap of the TiOxNyfilms was calculated from the linear fit of the
lin-ear portion from the (αhν)1/2vs. hν plot at the optical absorption edge.
The obtained results indicate that the energy band-gap (Eg) gradually
decreases from 3.18 eV (for thefilms with 0.6 at.% content of N) to 1.09 eV (for thefilms with 24.2 at.% content of N).Fig. 3presents the de-pendencies of nitrogen content and energy bandgap on the m-HiPIMS
Table 1
Atomic content and energy bandgap of six TiOxNythinfilms deposited at various values of m-HiPIMS pulse repetition frequency.
Sample S1 S2 S3 S4 S5 S6 Frequency (Hz) 750 1000 1150 1250 1400 1600 N (at.%) 0.6 12 15.6 17.2 22 24.2 O (at.%) 63.2 55.4 51.5 49.8 43.8 40.17 Ti (at.%) 36.2 32.6 32.8 33 34.2 35.6 Bandgap (eV) 3.18 2.35 1.72 1.65 1.15 1.09
pulse sequence repetition frequency values. The values of energy bandgap of the six TiOxNyfilms are also presented inTable 1.
The crystalline structure of the as-deposited and thermal annealed films was investigated by X-ray diffraction and Raman spectroscopy measurements. The X-ray diffraction patterns acquired at grazing inci-dence for the as-depositedfilms showed no diffraction peaks. Incorpo-ration of right amount of nitrogen and annealing treatment at high temperature might lead to formation of oxynitride crystalline phases. Therefore, we annealed the depositedfilms for one hour in nitrogen at-mosphere at 400 °C. However, only the sample S1 showed presence of diffraction peaks, which correspond to rutile (1 1 1) and (2 0 0) TiO2
phase (Fig. 4). The annealing treatment did not have the same effect for thefilms with larger content of nitrogen, which remained amor-phous (with short range order structures). We believe that formation of crystalline phases in thefilms with high content of nitrogen is ham-pered by the large number of interstitial nitrogen atoms.
The structure of TiOxNythinfilms was investigated also by Raman
spectroscopy. The Raman spectra of the as-deposited samples con-firmed that the samples present almost amorphous structures, The Raman spectra of the samples after thermal annealing (shown in Fig. 5) confirmed the results of the XRD investigation. Intense Raman peaks appear only for the sample with small amount of nitrogen (S1), indicating a high degree of crystallinity. The typical Raman modes at 145, 396, 445, 496, 613 and 640 cm−1 are assigned to the Eg(A),
B1g(A), Eg(R), B1g(A), A1g(R), and Eg(A) modes in anatase and rutile
structures[20]. The strongest modes at 145, 613 and 640 cm−1indicate
that anatase and rutile phases with a long-range order have been ob-tained only for the sample S1.
The X-ray photoelectron spectroscopy measurements have been done to investigate the chemical composition and bonding of nitrogen in the annealed TiOxNyfilms. The stoichiometry of the deposited films
has been evaluated from integrated peak areas of Ti-2p, O-1s and N-1s XPS signals with appropriate correction factors. The results confirmed the atomic composition of thefilms found by the RBS investigation (Table 1).Fig. 6shows the structure of Ti 2p core electron levels in the annealedfilm S2 containing around 12 at.% nitrogen. The Ti-2p signal has been resolved into four contributions: two peaks at 458.5 eV and 464.2 eV attributed to Ti4 +\\O
2
2–bonds and two peaks at 456.3 eV
and 462.3 eV assigned to Ti+ 3\\N3−bonds[21]. Unfortunately, it is
hard to discriminate between titanium nitride and titanium metallic signals due to closeness of the corresponding electronic states[15,22]. The N-1s signal (not shown) has shown a strong peak at 396.4 eV corre-sponding to N\\Ti bonds and a weak peak at 399.8 eV corresponding to adsorbed N2[23]. In conclusion, the XPS measurements indicate that
most of nitrogen is chemically bounded to titanium atoms with possible excess of titanium atoms.
The atomic force microscopy measurements revealed no noticeable differences in surface morphology of the either as-deposited or annealed thinfilms, the root mean square roughness having values
Fig. 2. UV–VIS transmission spectra of TiOxNythinfilms deposited on quartz substrates.
Fig. 3. Content of N and energy bandgap of TiOxNythinfilms deposited at various values of
HiPIMS pulse sequence repetition frequency.
Fig. 4. The XRD pattern of sample S1 after annealing. The peaks at 38 and 40° are attributed to formation of rutile nanocrystalites in thefilm structure[19].
Fig. 5. Raman spectra of TiOxNythinfilms deposited by m-HiPIMS at various values of
repetition frequency. The micro-Raman spectra have been collected at room temperature with a Raman spectrometer operating with the 632 nm spectral line in backscattering geometry in the range of 100–1000 cm−1.
around 0.5 nm have been found for all the investigated TiOxNyfilm
surfaces.
3.2. Photocatalytic activity
The visible-light photocatalytic activity of the as-deposited TiOxNy
thinfilms has been estimated firstly by measurements of the efficiency of thefilms irradiated by visible light (λ N 400 nm) for photocatalytic degradation of methylene blue (MB) molecules in an aqueous solution. For all the as-depositedfilms, the photocatalytic activity for MB mole-cule degradation was weak. Then, the TiOxNyfilms were annealed for
one hour in nitrogen atmosphere at 400 °C in order to improve the film crystalline order and enhance their photocatalytic activity. The an-nealing treatment resulted in a noticeable improvement of photocata-lytic activity only for thefilm with the smallest content of nitrogen (S1).Fig. 7is showing comparatively the time evolution of MB concen-tration as result of photocatalytic degradation of MB on the visible-light irradiatedfilm S1 before and after annealing treatment. As resulted from the X-ray diffraction investigation of the annealed sample S1, this improvement of photocatalytic activity can be attributed to the im-provement of the crystalline order in thefilm. Indeed, the X-ray diffrac-tion pattern of the sample S1 after annealing treatment shows presence
of rutile (1 1 1) and rutile (2 0 0) diffraction peaks (Fig. 4). Incorporation of right amount of nitrogen and annealing treatment at higher temper-ature might lead to formation of oxynitride crystalline phases.
The photocatalytic activity for water molecule splitting of the annealed TiOxNyfilms deposited on copper substrates used as
photo-anodes in a photoelectrochemical (PEC) cell has been evaluated. The PEC response of the as-deposited and annealedfilms with large content of nitrogen was extremely weak. Moreover the as-depositedfilms were not stable during PEC measurements, they being etched by the NaOH solution. Compared to the annealed TiOxNythinfilms with large content
of nitrogen, the annealedfilm S1 (with 0.6 at.% of nitrogen) showed a much better photocatalytic activity in visible light.Fig. 8shows the com-parison between PEC response of the annealedfilms S1 and S2 illumi-nated by time-chopped light from a xenon lamp simulating the sunlight[18]. For thefilm S2 (12 at.% nitrogen), it has been observed a very small photocurrent (defined as the difference between dark and light current). The low photocatalytic activity of thefilms with high con-tent of nitrogen could be caused by the high density of defects (e.g. ox-ygen vacancies). For thefilm S1, a maximum PC density of 0.3 mA/cm2
was obtained for a bias of 0.6 V vs. Ag/AgCl, which shows the good pho-tocatalytic activity for water splitting of thisfilm. Also inFig. 8, the PEC response for photocatalytic water splitting of the TiOxNythinfilms S1 Fig. 6. The XPS signal corresponding to Ti 2p states in thefilm S2 (12 at.% nitrogen). The
base line has been extracted from the XPS signal.
Fig. 7. Time variation of MB concentration in aqueous solution due to photocatalytic decomposition of MB under visible light radiation (λ N 400 nm, power density = 120 mW/cm2
) showing improvement of photocatalytic activity of sample S1 as result of post-deposition annealing treatment.
Fig. 8. Chopped light polarization curves of the annealed TiOxNythinfilms S1 (0.6 at.%
nitrogen), S2 (12 at.% nitrogen) and a TiO2thinfilm deposited by m-HiPIMS in the same
experimental conditions as S1, excepting the composition of the working gas, which did not contain oxygen. The incident UV lightflux of 100 mW/cm2
, electrolyte 0.1 M NaOH, scan rate of 50 mV s−1.
Fig. 9. Incident-Photon-to-Current-Efficiency (IPCE) measured at various values of biasing potential (vs. Ag/AgCl) for the annealed TiOxNythinfilms S1 (0.6 at.% nitrogen).and S2
and S2 is compared with the PEC response of a pure TiO2thinfilm
(de-posited by m-HiPIMS in the same experimental conditions as S1, ex-cepting the composition of the working gas, which did not contain oxygen). The PEC response of the TiO2film was much weaker than the
PEC response of S1, but larger than the PEC response of S2. This means that incorporation of small amount of nitrogen in TiO2thinfilms is
ben-eficial for the film photocatalytic activity, while incorporation of larger amount of nitrogen is detrimental, probably due to the large number of defect states (low crystalline order) associated with incorporation of large amounts of nitrogen.
The wavelength dependence of the Incident-Photon-to-Current-Ef-ficiency (IPCE) for the films S1 and S2 was measured at various values of biasing potential (vs. Ag/AgCl). The IPCE was calculated with[18]: IPCEð Þ ¼λ hλ c Jphð Þλ
e P λð Þð Þ% ð1Þ
where Jph(λ) is the photocurrent density, P(λ) the incident power
den-sity,λ the wavelength of the incident light, h the Planck constant (6.62 × 10−34J.s), c the light velocity (3 × 108m/s), and e the
elemen-tary charge (1.6 × 10−19C).Fig. 9shows the variation of IPCE versus wavelength for the samples S1 and S2. It is noticed that IPCE of the ple S2 was very low, irrespective of the wavelength value. For the sam-ple S1, the IPCE values show very good quantum efficiency for UV radiation (about 60% at a biasing potential of 0.5 V) and a cutoff (fast drop of values) at 400 nm. This means that the incorporation of nitrogen in thefilm S1 resulted in a slight decrease of the bandgap of this film and, thus, improved slightly its photocatalytic activity in visible light. Therefore, its good PEC response in sun light (simulated by the xenon lamp) is owed mainly to the UV radiation.
4. Conclusion
We have investigated the capability of HiPIMS deposition technique to synthesize TiOxNythinfilms with visible light photocatalytic activity
for water splitting. To increase the deposition rate and probabilities of chemical reactions in the deposition chamber, we operate the HiPIMS discharge in multiple pulse mode (m-HiPIMS), i.e. a pulse with 15μs in width was split in three micro-pulses with 5μs in width. It has been shown that the concentration of N atoms in the m-HiPIMS depos-ited TiOxNythinfilms can be controlled by pulse sequence repetition
frequency provided the oxygen content in the working gas is low (massflow ratio of 0.16 sccm for O2, 2 sccm for N2and 50 sccm for
Ar) and the substrates were kept at room temperature. Thin TiOxNy
films with nitrogen content ranged between 0.6 at.% and 24 at.% were obtained by increasing the m-HiPIMS repetition frequency from 750 Hz to 1600 Hz. The as-deposited TiOxNyfilms had short-range
order corresponding to anatase and rutile structures and showed weak photocatalytic activity. For thefilm with very low content of nitro-gen, a post deposition annealing treatment resulted in a noticeable im-provement of crystalline order and photocatalytic activity. Thefilms with higher content of nitrogen showed no improvement in crystalline order or photocatalytic activity as result of the annealing treatment. When comparing thefilm activity for water splitting, a pure TiO2thin
film showed much weaker activity than the TiOxNyfilm with low
con-tent of nitrogen (0.6 at.%) and higher activity than the TiOxNyfilms
with higher content of nitrogen (N12 at.%). Therefore, we conclude that incorporation of small amount of nitrogen in TiO2thinfilms is
ben-eficial, while incorporation of larger amount of nitrogen is detrimental for thefilm photocatalytic activity in visible light. These results point to the important role of crystalline order for improving of the photocat-alytic activity of semiconductors in visible light, besides the role of a
smaller energy bandgap. Although we did not obtain crystalline TiOxNy
films in this work, we proved that the amount of nitrogen incorporated into thefilms deposited by m-HiPIMS can be easily controlled. Post de-position treatments as the thermal annealing in nitrogen might improve thefilm crystalline order and photocatalytic activity and this will be subject for further investigations.
Acknowledgements
This work was supported by Joint Research Projects PN-II-ID-JRP-2012-RO-FR-0161.
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